Synthesis And Characterization Of Phosphate Cathode Materials Of Aqueous Rechargeable Sodium Ion Battery

Aqueous rechargeable sodium ion batteries（ARSIBs）featuring high safety,low cost,and environmental friendliness show great potential in the field of large-scale energy storage.The cathode materials represent the key to the further application of ARSIBs.Due to the large radius and slow diffusion kinetics of sodium ions,the structure of cathode materials is crucial for fast reversible insertion-desertion of sodium ions.The main research object of this thesis is based on the NASICON-type（Na-Super-Ionic-Conductor）NaTi₂（PO₄）₃ and Na₃V₂（PO₄）₃ which have large open diffusion channels for sodium-ion diffusion and storage.To further improve the performances of these two types of cathode materials,for the former,Mn²⁺or V³⁺with high redox potential was doped into NaTi₂（PO₄）₃ structure to elevate the charge-discharge voltage of the NaTi₂（PO₄）₃.And for the latter,the cycling stability of Na₃V₂（PO₄）₃ was improved by incorporating Mg²⁺or Al³⁺ions.The main results and conclusions of this thesis are as follows:（1）Na₃MnTi（PO₄）₃ material.Na₃MnTi（PO₄）₃ materials were prepared via different methods:sol-gel method,solid state reaction method,co-precipitation method and spray-drying method.Na₃MnTi（PO₄）₃ prepared via sol-gel method and spray-drying method has excellent electrochemical activity.The redox potential of Na₃MnTi（PO₄）₃ material is 0.71/0.44 V（vs.SCE）,and the average charge/discharge voltage of NaTi₂（PO₄）₃-Na₃MnTi（PO₄）₃ full-cell is 1.40 V.It was found that adding 0.1M Mn²⁺ions to the electrolyte can increase the specific capacity of Na₃MnTi（PO₄）₃.Galvanostatic charge/discharge testing shows that the Na₃MnTi（PO₄）₃ samples experienced structural change.Increasing the Mn²⁺-doping amount can further increase the specific capacity of the material.Meanwhile,increasing the sodium content can significantly increase the initial specific discharge capacity of the cathode,with the material Na_3.5Mn_1.25Ti_0.75（PO₄）₃ demonstrating the highest initial capacity at 60 mAh g^-1.（2）Na₂VTi（PO₄）₃ material.Na₂VTi（PO₄）₃ materials were also prepared via the sol-gel method.This material is bi-functional Na₂VTi（PO₄）₃ with two redox potential windows of-0.87/-0.68 V and 0.29/0.51 V（vs.SCE）,respectively,enables a symmetric cell with a well-defined discharge plateau at 1.2 V.The initial specific capacity of Na₂VTi（PO₄）₃ cathode materials is 47.7 mAh g^-1 and the capacity retention is 79.7%after 100 cycles.（3）Na_3+xV₂-_xMg_x（PO₄）₃ material.Na_3+xV₂-_xMg_x（PO₄）₃（x=0,0.03,0.06,0.12,0.25）materials were prepared via the sol-gel method.The effect of Mg substitution on crystal structure was examined by XRD.The results indicate that low level Mg-doping did not change the original crystal structure of Na₃V₂（PO₄）₃.Galvanostatic charge/discharge test shows that doped samples have significantly improved cycling stability.Especially,the x=0.06 sample shows more than 95%capacity retention after300 cycles.Ex-situ XRD reveals that the phase transition occurred in the first cycle,then the structure maintained during the following cycles,which improved the cycling stability of Na_3+xV₂-_xMg_x（PO₄）₃.（4）Na₃V₂-_xAl_x（PO₄）₃ material.Na₃V₂-_xAl_x（PO₄）₃（x=0,0.015,0.03,0.06,0.12）materials with carbon coating were prepared via sol-gel method.XRD measurements indicated that the original crystal structure of Al-doped samples did not change overall.However,the lattice parameters of doped sample decreased,due to the smaller radius of Al³⁺ion than that of V³⁺ion.Compared with the pristine Na₃V₂（PO₄）₃,the Na₃V_1.97Al_0.03（PO₄）₃ exhibits significantly improved cycling stability with 93%capacity retention after 350 cycles.Intriguingly,this new material presents different electrochemical behaviors at different cutoff voltages.By controlling the cutoff voltage within 0-0.6 V（vs.SCE）,a remarkably high capacity retention of 81.3%after 800cycles is achieved.